The present invention generally relates to rare earth oxide coated phosphors useful in field emission display devices and vacuum fluorescent display devices and a process for preparing the coated phosphors, and particularly, to rare earth oxide coated sulfide phosphors having improved cathodoluminescence properties and a process for preparing them.
The operating characteristics of field emission displays (FED) and vacuum fluorescent displays (VFD) place many demands upon cathodoluminescent phosphors. In particular, the lower operating voltages used in FEDs compared to cathode ray tubes (CRT) result in smaller penetration depths and reduced luminous efficiency and brightness. This reduction in luminous efficiency can be compensated for by using higher drive currents or current densities and taking advantage of the longer pixel dwell time (e.g., 30 xcexcs) used in FEDs.
However, the use of higher current densities accelerates coulombic aging, e.g., degradation by electron bombardment, as well as degradation of the phosphor by interactions with the residual atmosphere in the vacuum devices. The phosphor degradation can lead to poisoning of the field emission cathodes. Further, since low voltages are employed, operation of the device becomes especially challenging in view of the resistance produced by contaminations such as the surface dead layer. The foregoing shows that there exists a need for low voltage phosphors having both high efficiency and chemical stability.
Furthermore, the phosphors undergo a baking process during the manufacture of VFDs and FEDs during which the surface of the phosphor is oxidized to some extent. The oxidation causes deterioration of the crystallinity of the phosphor surface, and consequently, the oxidized surface decomposes or degrades during operation of the devices. In addition, certain phosphors, e.g., as sulfide phosphors, undergo an electron stimulated decomposition which generates gases such as gases containing S, SO, and/or SO2. These gases are harmful, and they can reduce the electron emission efficiency of the cathode. Thus, there further exists a need for phosphors, particularly sulfide phosphors, that have increased resistance to oxidative and/or electron stimulated decomposition.
These and other advantages of the present invention, as well as additional inventive features, will become apparent from the description of the present invention provided herein.
Many of the foregoing needs have been fulfilled by the present invention which provides a surface coated phosphor comprising a thin coating of a rare earth oxide disposed on an uncoated phosphor. The present invention further provides a process for preparing the coated phosphor comprising contacting the uncoated phosphor with a rare earth hydroxide gel solution to obtain a rare earth hydroxide gel coated phosphor and heat treating the rare earth hydroxide gel coated phosphor.
While the invention has been described and disclosed below in connection with certain embodiments and procedures, it is not intended to limit the invention to those specific embodiments. Rather it is intended to cover all such alternative embodiments and modifications as fall within the spirit and scope of the invention.